1. Field of the Invention
The present invention relates to Pb-free piezoelectric ceramics of alkali-containing niobate perovskite structure, as well as a piezoelectric element containing such piezoelectric ceramics.
2. Description of the Related Art
Piezoelectric ceramics are used as piezoelectric elements. Piezoelectric elements are used as sensor elements and generation elements, among others, by applying the piezoelectric effect of converting mechanical energy to electrical energy. Piezoelectric elements are also used as vibrators, sound generators, actuators, ultrasonic motors, etc., that apply the reverse-piezoelectric effect of converting electrical energy to mechanical energy. In addition, piezoelectric elements are used as circuit elements, vibration control elements, etc., by combining the piezoelectric effect and reverse-piezoelectric effect.
In general, a piezoelectric element has a structure comprising layered piezoelectric ceramic sheets and internal electrodes placed between the layers. A piezoelectric element has two terminals and its internal electrodes are connected alternately to the different terminals. This way, voltage is applied to each piezoelectric ceramic layer when voltage is applied between the terminals.
For high-performance piezoelectric ceramics, PZT material expressed by the composition formula Pb(Zr,Ti)O3—PbTiO3, and PLZT material expressed by the composition formula (Pb,La)(Zr,Ti)O3—PbTiO3, are widely known. However, both of these piezoelectric ceramics, while offering high piezoelectric characteristics, contain Pb which imposes significant burdens on the environment.
Among Pb-free piezoelectric ceramics that do not contain Pb, piezoelectric ceramics having a perovskite structure of alkali-containing niobate (refer to Patent Literatures 1 to 7 and Non-patent Literatures 1 and 2) or of barium titanate (refer to Patent Literature 8) are known as offering relatively good performance.
In particular, Patent Literature 7 discloses piezoelectric ceramics offering improved piezoelectric characteristics, achieved by dispersing a sub phase containing the composition K3Nb3O6Si2O7 in the main phase of alkali-containing niobate perovskite structure and thereby producing a dense structure constituted by grains of uniform size.